Epitaxy is used in the semiconductor industry to grow a layer of semiconductor material on top of a single-crystalline semiconductor substrate in such a way that the crystal lattice is maintained. Common examples are the growth of an epitaxial silicon layer on top of a silicon substrate or the growth of a SiGe alloy layer on top of a silicon substrate. This can be done by chemical vapor deposition (CVD) in a proper environment. The wafer is heated to a suitable temperature and gases containing the required components are passed over the substrate, which usually is a silicon wafer. To avoid growth on the walls of the reactor vessel it is advantageous to heat just the wafer and to keep the reactor walls at a relatively low temperature.
Common epitaxial reactors used in the semiconductor industry consist of a transparent quartz reactor chamber through which the gas is passed. Inside the quartz chamber the substrate is located on top of a susceptor. The wafer and susceptor are irradiated by high intensity lamps and heated to the desired temperature. As the quartz walls of the reactor chamber absorb comparatively little radiation they can easily be cooled. The temperature of the susceptor is measured and controlled; it is assumed that the wafer has approximately the same temperature as the susceptor.
FIGS. 1a and 1b illustrate a simple case. A susceptor 10, usually a graphite disk with a SiC coating, has a pocket 11 machined in its top surface to hold a wafer (not shown). The temperature of the susceptor 10 is measured by a pyrometer 12 aimed at the backside of the susceptor 10. The pyrometer can also be aimed at the top surface of the wafer, directly measuring the wafer temperature. However, since the emissivity of the wafer surface is not constant, especially when patterned wafers have to be deposited, this is not a good method to measure the wafer temperature. In order to improve the uniformity of the deposited layer the susceptor 10 can rotate during the deposition process.
FIGS. 2a, 2b and 2c show a more complicated case. A susceptor 20 consists of two parts: a rotating inner part 21 and a stationary outer part 22 as shown in FIG. 2a. The rotating inner part 21 holds a wafer (not shown) in a pocket 23 and rotates. The stationary outer part 22 (e.g., a ring) of the susceptor 20 is made of the same material as the rotating inner part 21, but does not rotate. This is done to enable a temperature measurement to be done with thermocouple 28 (shown in cross section in FIG. 2c).
A small pocket 25 centered at the backside of the susceptor 20 (shown in cross section in FIG. 2b) is intended to receive the tip of the thermocouple 24 as shown in FIG. 2c. The thermocouple 24 rotates together with the rotating inner part 21. The stationary outer part 22 holds one or more thermocouples 28 via a thermocouple bore 26 to measure the temperature at the wafer edge. All the thermocouples 24 and 28 are tied to a temperature control system 27 allowing for a very accurate control of the temperature at the center and the edge of the susceptor 20.
While these arrangements have proven useful, significant shortcomings exist with these arrangements.